Temperature effects on the diversity of soil heterotrophs and the δ13C of soil-respired CO2

We measured the respiration rates, δ13C of respired CO2, and microbial community composition in root-free bulk soils incubated at 4, 22 and 40°C. The soils were obtained from the Duke Forest Free-Air CO2 Enrichment (FACE) experiment where organic carbon in soils sampled from the elevated CO2 plots contained a unique 13C label that was derived from FACE fumigation. The CO2 produced by soil heterotrophs at 4°C was 2.2 to 3.5‰ enriched in 13C relative to CO2 respired at 22 and 40°C and was similarly enriched relative to bulk soil carbon. There was no isotopic difference between CO2 produced at 22 and 40°C. Respiration rates increased exponentially with temperature from 0.25 mg CO2 g soil−1 d−1 at 4°C to 0.73 mg CO2 g soil−1 d−1 at 40°C. Microbial community composition, as measured by the differences in populations of morphology types, differed across the temperature range. Only eight of 67 microbial morphology types were common to all three incubation temperatures, while six types were unique to 4°C soil, 17 to 22°C soil and 18 to 40°C soil. Species richness, approximated from morphology type, was significantly lower at 4°C than at 22 and 40°C. This change in microbial community structure from 4 to 22 and 40°C caused a shift in mineralizable carbon pools, resulting in a shift in the isotopic composition of CO2 respired at the low temperature.